flexible ac transmission systems (1)

Due to the influence of FACTS-devices on wide system areas, especially for power flow and damping control, an exchange of system information with the FACTS-controllers is required.. A wi

Power Systems

Xiao-Ping Zhang, Christian Rehtanz, Bikash Pal

Flexible AC Transmission Systems: Modelling and Control

Xiao-Ping Zhang, Christian Rehtanz, Bikash Pal

Flexible AC

Transmission Systems: Modelling and Control

With 156 Figures

Dr Xiao-Ping Zhang

University Warwick

School of Engineering

Coventry CV4 7AL

United Kingdom

x.p.zhang@warwick.ac.uk

Dr Christian Rehtanz

ABB Corporate Research China

Universal Plaza, 10 Jiuxianqiao Lu

Chaoyang District

Beijing, 100016

P.R China

christian.rehtanz@ieee.org

ISBN-10 3-540-30606-4 Springer Berlin Heidelberg New York

ISBN-13 978-3-540-30606-1 Springer Berlin Heidelberg New York This work is subject to copyright All rights are reserved, whether the whole or part of the material

is concerned, specifi cally the rights of translation, reprinting, reuse of illustrations, recitation, broad-casting, reproduction on microfi lm or in other ways, and storage in data banks Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Sprin-ger Violations are liable to prosecution under German Copyright Law.

Springer is a part of Springer Science+Business Media

springer.com

© Springer-Verlag Berlin Heidelberg 2006

Printed in Germany

The use of general descriptive names, registered names, trademarks, etc in this publication does not imply, even in the absence of a specifi c statement, that such names are exempt from the relevant pro-tective laws and regulations and therefore free for general use.

Typesetting: Digital data supplied by editors

Final processing by PTP-Berlin Protago-TEX-Production GmbH, Germany

Cover-Design: deblik, Berlin

Printed on acid-free paper 89/3141/Yu – 5 4 3 2 1 0

Library of Congress Control Number: 2005936513

Bikash Pal

Imperial College London Dept of Electrical & Eelctronic Engineering Exhibition Road

London SW7 2BT United Kingdom

b.pal@imperial.ac.uk

The electric power industry is undergoing the most profound technical, economic and organisational changes since its inception some one hundred years ago This paradigm change is the result of the liberalisation process, stipulated by politics and followed up by industry For many years the electric power industry was char-acterized by a vertically integrated structure, consisting of power generation, transmission/distribution and trading The liberalisation process has resulted in the unbundling of this organizational structure Now generation and trading are organ-ised in separate business entities, subject to competition, while the transmis-sion/distribution business remains a natural monopoly Since the trading of elec-tric energy happens on two levels, the physical level and the contractual level, it has to be recognized that these two levels are completely different However for understanding the electricity market as a network based industry both levels have

to be considered and understood The fundamental properties of electric energy are as follows:

• Electricity always needs a network for transportation and distribution

• Electricity cannot be stored in a substantial amount, hence production and con-sumption have to be matched at each instant of time

• The physical transport of electricity has nothing to do with the contracts for trading with electricity

The role of the electric network is of prime importance within the electric energy business Its operation is governed by physical laws The electric network has a fixed structure consisting of different voltage levels; the higher levels are for transmission purposes whereas the lower levels are used for the distribution tasks Each network element has a finite capacity, limiting the amount of electricity to be transported or distributed As a consequence of the liberalisation process the op-eration of the networks has been pushed closer towards its technical limits Hence the stress on the system is considerably bigger than in the past The efficient use of all network elements is of prime interest to the network operator because the cost constraints have also become much tighter than in the past Recognizing that the operation of a large electric network is a complex and challenging engineering task, it becomes evident that the cost constraints increase the operational complex-ity considerably The bigger the interconnected network becomes the more flexi-bility is required with respect to the cross border trading of electricity Simultane-ously the complexity of operational problems increases due to voltage, angle and frequency stability problems

VI Foreword

The traditional planning approaches for power networks are undergoing a reen-gineering The long lasting experience with the power flowing purely from the generation plants to the customers is no longer valid Growing volatility and in-creasingly unpredictable system behaviour requires innovative equipment to han-dle such situations successfully Keeping in mind that the interconnected power networks have been designed such that each network partner may contribute with reserve power in case of emergency, the trend is now towards extensive cross bor-der energy trading Another fundamental development is the construction of micro grid on the distribution level The introduction of dispersed generation close to the customers changes the functionality and the requirements of the distribution net-works The grid operator is requested to provide network access to any interested stakeholder in a transparent and non-discriminatory manner So, while in the past the power flow in distribution networks was unidirectional, now the system must handle bidirectional power flows This allows the distribution network to take on more and more the function of a balancing network At the same time, the capacity

of individual elements may not be sufficient to cope with the resulting power flow situations

Summarizing the current developments, it must be noticed that both planning and operation of electric networks are undergoing fundamental and radical changes in order to cope with the increased complexity of finding economic and reliable network solutions The operation of the transmission and distribution net-works will be closer to their physical limits The necessity to design electric power networks providing the maximal transmission capacity and at the same time re-sulting in minimal costs is a great engineering challenge Innovative operational equipment based on power electronics offers new and powerful solutions Com-monly described by the term 'Flexible AC Transmission Systems' or 'FACTS-devices', such equipment has been available for several years, but has still not been widely accepted by all grid operators for several reasons

The introduction of innovative equipment has a great impact on the operation

A more flexible transmission or distribution system may cause new problems dur-ing normal or disturbed operatdur-ing states Furthermore, the proper understanddur-ing of innovative equipment is also an educational problem because there is not much experience reported so far with this innovative equipment

On the other hand, the opportunities for new solutions are substantial and portant FACTS-devices can be utilized to increase the transmission capacity, im-prove the stability and dynamic behaviour or ensure better power quality in mod-ern power systems Their main capabilities are reactive power compensation, voltage control and power flow control Due to their controllable power electron-ics, FACTS-devices always provide fast control actions in comparison to conven-tional devices like switched compensation or phase shifting transformers with me-chanical on-load tap changers

This book offers a concise and modern presentation of the timely and important topic of flexible AC transmission networks There is no doubt that these innova-tive FACTS-devices will find a definite place in transmission and distribution networks The complete description of the functionality of such devices is sup-ported with extensive mathematical models, which are required when planning the

Foreword VII

use of this type of equipment in electrical networks The first part of the book deals with the modeling of single and multi-converter FACTS-devices in single and three-phase power flow studies and optimal power flow solutions

The in depth discussion of the operational and controlling aspects in the second part of the book makes it a most valuable compendium for the design of future electric networks Without a complete and powerful solution of the control prob-lems, the FACTS-devices will not find their application in power systems because they have to operate in normal and contingency situations in a reliable and eco-nomic way System security must not be weakened by the FACTS-devices, even if the system is operated closer to its limits The control speed of the FACTS-devices can only be utilized, if they are first given higher priority from the operator, then designed to react in a coordinated but autonomous manner in dynamic or even contingency situations A novel and original control strategy based on the autonomous control theory fulfilling these requirements is presented in the book Due to the influence of FACTS-devices on wide system areas, especially for power flow and damping control, an exchange of system information with the FACTS-controllers is required A wide area control scheme is introduced and ap-plied for power flow control The dynamics of FACTS-devices provide effective damping capability Inter-area oscillations require wide area system supervision and a wide area control scheme For this application time delays in the wide area control loop play a significant role in the controller design Based on detailed modeling, an innovative approach is presented considering this time delay, making wide area damping control feasible Only with such a control scheme, FACTS-devices can be applied beneficially in the future

Based on the authors' extensive experience, this book is of greatest importance for the practical power engineers for both planning and operational problems It provides a deep insight into the use of FACTS-devices in modern power systems Although the technology of modern power electronics will change very quickly, the results presented in this book are sustainable and long lasting The combina-tion of theoretical and practical knowledge from the internacombina-tional team of authors from academia and industry provides an invaluable contribution for the future ap-plication of FACTS-devices I am convinced that this book will become a standard work in modern power engineering It will serve equally as a text book for univer-sity students as well as an engineering reference for planning and operation of modern power systems

Prof Dr.-Ing Edmund Handschin Dortmund, Germany, 2005

Electricity market activities and a growing demand for electricity have led to heavily stressed power systems This requires operation of the networks closer to their stability limits Power system operation is affected by stability related prob-lems, leading to unpredictable system behavior Cost efficient solutions are pre-ferred over network extensions In many countries, permits to build new transmis-sion lines are hard to get, which means the existing network has to be enforced to fulfill the changing requirements

Power electronic network controllers, the so called FACTS-devices, are well known having several years documented use in practice and research Several kinds of FACTS-devices have been developed Some of them such as the Thyris-tor based Static Var CompensaThyris-tor (SVC) are a widely applied technology; others like the Voltage Source Converter (VSC) based Static Compensator (STATCOM)

or the VSC-HVDC are being used in a growing number of installations world-wide The most versatile FACTS-devices, such as Unified Power Flow Controller (UPFC), although still confined primarily to research and development applica-tions, have the potential to be used widely beyond today's pilot installations

In general, FACTS-devices can be utilized to increase the transmission capac-ity, the stability margin and dynamic behavior or serve to ensure improved power quality Their main capabilities are reactive power compensation, voltage control and power flow control Due to their controllable power electronics, FACTS-device provide always a fast controllability in comparison to conventional FACTS-devices like switched compensation or phase shifting transformers Different control op-tions provide a high flexibility and lead to multi-functional devices

To explore the capabilities of FACTS-devices, a specific operation and control scheme has to be designed Fundamental to their operation and control is their proper modeling for static and dynamic purposes The integration of FACTS-devices into basic tools like power flow calculation and optimal power flow (OPF)

is mandatory for a beneficial system operation Due to the wide area and dynamic impact of FACTS-devices, a pure local control is desired, but is not sufficient in many cases The requirements for normal and emergency operation have to be de-fined carefully A specific control design has to address these different operational conditions This book introduces the latest results of research and practice for modeling and control of existing and newly introduced FACTS-devices

X Preface

Motivation

This book is motivated by the recent developments of FACTS-devices Numerous types of FACTS-devices have been successfully applied in practical operation Some are still in the pilot stage and many are proposed in research and develop-ment From practical experience it has been seen that the investment into FACTS-devices, in most of the cases, only pays off by considering their multi-functional capabilities, particularly in normal and emergency situations This requires a three-phase modeling and a control design addressing both normal and emergency conditions which, in most of the cases, uses wide area information The recent re-sults and requirements for both modeling and control have motivated this book

Focus and Target

The focus and target of this book is to emphasize advanced modeling, analysis and control techniques of FACTS These topics reflect the recent research and devel-opment of FACTS-devices, and foresee the future applications of FACTS in power systems The book comprehensively covers a range of power system con-trol problems like steady state voltage and power flow concon-trol, voltage and reac-tive power control, voltage stability control and small signal stability control using FACTS-devices

Beside the more mature FACTS-devices for shunt compensation, like SVC and STATCOM, and series compensation, like TCSC and SSSC, the modeling of the latest FACTS-devices for power flow control, compensation and power quality (IPFC, GUPFC, VSC HVDC and Multi-VSC-HVDC, etc.) is considered for power system analysis The selection is evaluated by their actual and future practi-cal relevance The multi-control functional models of FACTS-devices and the ability for handling various internal and external operating constraints of FACTS are introduced In addition, models are proposed to deal with small or zero imped-ances in the voltage source converter (VSC) based devices The FACTS-device models are implemented in power flow and optimal power flow (OPF) cal-culations The power flow and OPF algorithms cover both single-phase models and especially three-phase models Furthermore the unbalanced continuation power flow with FACTS is presented

The control of FACTS-devices has to follow their multi-functional capabilities

in normal and emergency situations The investment into FACTS is normally jus-tified by the increase of stability and primarily by the increase of transmission ca-pability Applications of FACTS in power system operation and control, such as transfer capability enhancement and congestion management, are used to show the practical benefits of FACTS devices

A comprehensive FACTS-control approach is introduced based on the require-ments and specifications derived from practical experience The control structure

is characterised by an autonomous system structure allowing, as far as possible, control decisions to be taken locally, but also incorporating system wide informa-tion where this is required Wide Area Measurement System (WAMS) based con-trol methodologies, which have been developed recently, are introduced for the

Preface XI

first time in a book In particular, the real-time control technologies based on Wide Area Measurement are presented The current applications and future devel-opments of the Wide Area Measurement based control methodologies are also discussed As a particular control topic, utilizing the control speed of FACTS-devices, a special scheme for small-signal stability and damping of inter-area os-cillations is introduced Advanced control design techniques for power systems with FACTS including eigenvalue analysis, damping control design by the state-of-the art Linear Matrix Inequalities (LMI) approach and multiple damping con-troller coordination is presented In addition, the time-delay of wide area commu-nications, which is required for a system wide damping control, is considered These aspects make the book unique in its area and differentiate from other books on the similar topic The work presented is derived both from scientific re-search and industrial development, in which the authors have been heavily in-volved The book is well timed, addressing current challenges and concerns faced

by the power engineering professionals both in industries and academia It covers

a broad practical range of power system operation, planning and control problems

Structure

The first chapter of the book gives an introduction into nowadays FACTS-devices Power semiconductors and converter structures are introduced The basic designs

of major FACTS-devices are presented and discussed from a practical point of view The further chapters are logically separated into a modeling and a control part The modeling part introduces the modeling of single and multi-converter FACTS-devices for power flow calculations (Chapter 2 and 3) and optimal power flow calculations (Chapter 4) The extension to three phase models is given in chapter 5 This is fundamental for proper system integration for steady state bal-anced and unbalbal-anced voltage stability control or the increase of available trans-mission capacity

Chapter 6 and 7 present the steady state voltage stability analysis for balanced and unbalanced systems The increase of transmission capacity and loss reduction with power flow controlling FACTS-devices is introduced in chapter 8 along with the financial benefits of FACTS From these results it can be seen, that the bene-fits of FACTS can be increased by utilizing the fast controllability of FACTS to-gether with a certain wide area control scheme

The control part of the book starts with chapter 9 introducing a non-intrusive system control scheme for normal and emergency situations The chapter takes the view, that a FACTS-device should never weaken the system stability Based on this condition, the requirements and basic control scheme for FACTS-devices are derived Chapter 10 introduces an autonomous control system approach for FACTS-control, balancing the use of local and global system information and considering normal and emergency situations Due to the influence of FACTS-devices on wide system areas, especially for power flow and damping control, an exchange of information with the FACTS controllers is required A wide area con-trol scheme for power flow concon-trol is introduced in chapter 11 Only with wide area system information can the benefits of power flow control be achieved

XII Preface

The control options available with FACTS-devices can provide effective damp-ing capability Chapter 12 and 13 deal with small signal stability and the dampdamp-ing

of oscillations, which is a specific application area utilizing the control speed of FACTS The coordination of several FACTS damping controllers requires a for-mally introduced wide area control scheme This approach has to consider com-munication time delays carefully, which is a specific topic of chapter 13

Acknowledgements

The authors would like to thank Prof Edmund Handschin at the University of Dortmund, Germany for his support and encouragement to write this book Sig-nificant progress was made in the modeling of FACTS in power flow and optimal power flow analysis when Dr Zhang was working in Prof Handschin’s Institute

at the University of Dortmund, sponsored by the Alexander van Humboldt Foun-dation, Germany Subsequent work has been sponsored by the Engineering and Physics Sciences Research Council (EPSRC), UK Therefore, Dr Zhang would like to take the opportunity to acknowledge the support from the Alexander van Humboldt Foundation and the EPSRC

Dr Rehtanz would like to thank the following researchers for their contribu-tions to some of the chapters Chapter 8 is based on collaborative work with Prof Jürgen Haubrich, Dr Feng Li of RWTH, and Dr Christian Zimmer and Dr Alex-ander Ladermann of CONSENTEC GmbH, Aachen, Germany Dr Christian Becker, who was working with the University of Dortmund, and is now working with AIRBUS Deutschland GmbH, has contributed to chapter 10 Dr Mats Lars-son, Dr Petr Korba, and Mr Marek Zima, ABB, Switzerland have contributed with their work to chapter 11 Special thanks are given to Prof Dirk Westermann

of the Technical University Ilmenau, Germany for his useful contributions, inputs and comments to chapters 9 to 11

Dr Bikash Pal would like to thank Dr Balarko Chaudhuri of GE Global Re-search Lab, Bangalore and Mr Rajat Majumder, a PhD student at Imperial College for supporting him for the preparation of chapter 13 through simulation results The control design techniques presented in this chapter primarily comes from the research conducted by them under the supervision of Dr Pal at Imperial College

Dr Pal also expresses his gratitude to EPSRC (UK) and ABB for sponsoring this research at Imperial College Dr Pal is also thankful to Dr John McDonald of the Control and Power research group at Imperial College for proof reading chapters

12 and 13

The challenging task of writing and editing this book was made possible by the excellent co-operation of the team of authors together with a number of col-leagues and friends Our sincere thanks to all contributors, proofreaders, the pub-lisher and our families for making this book project happen

Xiao-Ping Zhang University of Warwick, Coventry, UK, 2005 Christian Rehtanz ABB China Ltd, Beijing, China, 2005 Bikash Pal Imperial College London, London, UK, 2005